As Sløk points out, the fortunes of online retailers and
department stores have slowly been diverging as more
shoppers are flocking online. So not only are shoppers
spending more on non-clothes items, but they are increasingly
looking for other sources of goods.

Another chart from Sløk shows that overall spending at
nonstore retailers (mostly online retailers) has been
increasing for the last several years, while department store
spending has been flat or declining:

Deutsche
Bank

So there you have it, the autopsy report for department stores in
chart form.